CELL DIVISIONS
337
D. Deviations from Typical Ratios of the Durations of
Various Developmental Periods
1. Development at Sublethal
Temperatures
Within the range of average spawning temperatures different
developmental stages have a closely similar temperature coefficient
(Qio) which ensures the constancy of the relative duration of different
developmental periods (cf. Dettlaff and Dettlaff, 1961). At optimal
temperatures the Q 10 value fluctuates around 2. When approaching the
higher limits of normal spawning temperatures the Q 10 begins to fall
(Bëlehradek, 1935), unequally for different developmental stages; at
higher temperatures it is the stage of cleavage which is the most sensitive.
Fig. 1 presents (as an example) a curve of temperature dependence in
t Qt miT\
130 «
«
j
j
j
j
j
I
no
1
110
100
90
30
70
60
50
40
10
12
14
16
18
20
22
24 °C
FIG. 1. Temperature dependence of duration of the period of the first cleavage
division (τ 0 ) of the sturgeon egg.
sturgeon eggs. The right hand of the curve turns upwards; i.e. with
increasing temperature, r 0 at first ceases to shorten and then even
lengthens. The same characteristics are shown by the curve of r 0
temperature-dependence in axolotl eggs (Skoblina, 1964). Because of
this, in the zone of upper sublethal temperatures, the duration of
development up to the end of gastrulation (τ π ) and neurulation (τ ΠΙ )
represents a smaller number of r 0 . Fig. 2 shows temperature dependence